arXiv subjects
Publications and source records attributed to J. Nieves.
We evaluate exclusive semileptonic decays of ground-state spin-1/2 doubly heavy charmed baryons. The decays are driven by a $c\to s,d$ transition at the quark level. Our form factors are consistent with Heavy Quark Symmetry constraints. The latter are valid in the limit of infinitely heavy quark mass at zero recoil.
Charmed baryon resonances that are generated dynamically are studied within a unitary baryon-meson coupled-channel model, which incorporates heavy-quark spin symmetry. The extension of the SU(3) Weinberg-Tomozawa chiral Lagrangian to SU(8) spin-flavor symmetry plus a suitable symmetry breaking is used. The model produces resonances with negative parity from s-wave interaction of pseudoscalar and vector mesons with $1/2^+$ and $3/2^+$ baryons. Some of our dynamically generated states can be readily assigned to resonances found experimentally, while others do not have a straightforward identification and require the compilation of more data and also refinements of the model.
We evaluate exclusive semileptonic decays of ground-state spin-1/2 and spin-3/2 doubly heavy $cb$ baryons driven by a $c\to s,d$ transition at the quark level. We check our results for the form factors against heavy quark spin symmetry constraints obtained in the limit of very large heavy quark masses and near zero recoil. Based on those constraints we make model independent, though approximate, predictions for ratios of decay widths.
We show that because of the multinucleon mechanism effects, the algorithm used to reconstruct the neutrino energy is not adequate when dealing with quasielastic-like events, and a distortion of the total flux unfolded cross section shape is produced. This amounts to a redistribution of strength from high to low energies, which gives rise to a sizable excess (deficit) of low (high) energy neutrinos. This distortion of the shape leads to a good description of the MiniBooNE unfolded CCQE-like cross sections published in Phys.Rev. D81 (2010) 092005. However, these changes in the shape are artifacts of the unfolding process that ignores multinucleon mechanisms.
With the aim of achieving a better and more complete understanding of neutrino interactions with nuclear targets, the coherent production of charged kaons induced by neutrinos and antineutrinos is investigated in the energy range of some of the current neutrino experiments. We follow a microscopic approach which, at the nucleon level, incorporates the most important mechanisms allowed by the chiral symmetry breaking pattern of QCD. The distortion of the outgoing (anti)kaon is taken into account by solving the Klein-Gordon equation with realistic optical potentials. Angular and momentum distributions are studied, as well as the energy and nuclear dependence of the total cross section.
Different experiments on hadron spectroscopy have long suspected the existence of several cascade states in the $1900-2000 {\rm MeV}$ region. They are usually labeled under the common name of $Ξ(1950)$. As we argue here, there are also theoretical reasons supporting the idea of several $Ξ(1950)$ resonances. In particular, we propose the existence of three $Ξ(1950)$ states: one of these states would be part of a spin-parity $(1/2)^{-}$ decuplet and the other two probably would belong to the $(5/2)^{+}$ and $(5/2)^{-}$ octets. We also identify which decay channels are the more appropriate for the detection of each of the previous states.
We study the semileptonic $b\to c$ decays of the lowest-lying triply-heavy baryons made from $b$ and $c$ quarks in the limit $m_b, m_c \gg Λ_\mathrm{QCD}$ and close to the zero recoil point. The separate heavy quark spin symmetries strongly constrain the matrix elements, leading to single form factors for $ccb\to ccc$, $bbc\to ccb$, and $bbb\to bbc$ baryon decays. We also study the effects on these systems of using a $Y$-shaped confinement potential, as suggested by lattice QCD results for the interaction between three static quarks.
We analyze the MiniBooNE CCQE $dσ/dT_μd\cosθ_μ$ data using a theoretical model that has proved to be quite successful in the analysis of nuclear reactions with electron, photon and pion probes. We find that RPA and multinucleon knockout turn out to be essential for the description of the MiniBooNE data. We show these measurements are fully compatible with former determinations of nucleon axial mass $M_A$, in contrast with several previous analyses, which have suggested an anomalously large value. We find, $M_A=1.08 \pm 0.03$ GeV. We also argue that the procedure, commonly used to reconstruct the neutrino energy for QE events from the muon angle and energy, could be unreliable for a wide region of the phase space, due to the large importance of multinucleon events.
The charged-current double differential neutrino cross section, measured by the MiniBooNE Collaboration, has been analyzed using a microscopical model that accounts for, among other nuclear effects, long range nuclear (RPA) correlations and multinucleon scattering. We find that MiniBooNE data are fully compatible with the world average of the nucleon axial mass in contrast with several previous analyses which have suggested an anomalously large value. We also discuss the reliability of the algorithm used to estimate the neutrino energy.
The anti-D meson self-energy is evaluated self-consistently, using unitarized coupled-channel theory, by computing the in-medium meson-baryon T-matrix in the C=-1,S=0 sector. The heavy pseudo-scalar and heavy vector mesons, anti-D and anti-D^*, are treated on equal footing as required by heavy quark spin symmetry. Results for energy levels and widths of D^- mesic atoms in 12C, 40Ca, 118Sn and 208Pb are presented. The spectrum contains states of atomic and of nuclear types for all nuclei. anti-D^0--nucleus bound states are also obtained. We find that, after electromagnetic and nuclear cascade, these systems end up with the anti-D bound in the nucleus, either as a meson or as part of a exotic anti-D N (pentaquark) loosely bound state.
We evaluate exclusive semileptonic decays of ground-state spin-1/2 doubly heavy charmed baryons driven by a $c\to s,d$ transition at the quark level. Our results for the form factors are consistent with heavy quark spin symmetry constraints which are valid in the limit of an infinitely massive charm quark and near zero recoil. Only a few exclusive semileptonic decay channels have been theoretically analyzed before. For those cases we find that our results are in a reasonable agreement with previous calculations.
We use a consistent SU(6) extension of the meson-baryon chiral Lagrangian within a coupled channel unitary approach in order to calculate the T-matrix for meson-baryon scattering in s-wave. The building blocks of the scheme are the pion and nucleon octets, the rho nonet and the Delta decuplet. We identify poles in this unitary T-matrix and interpret them as resonances. We study here the non exotic sectors with strangeness S=0,-1,-2,-3 and spin J=1/2, 3/2 and 5/2. Many of the poles generated can be associated with known N, Delta, Sigma, Lambda and Xi resonances with negative parity. We show that most of the low-lying three and four star odd parity baryon resonances with spin 1/2 and 3/2 can be related to multiplets of the spin-flavor symmetry group SU(6). This study allows us to predict the spin-parity of the Xi(1620), Xi(1690), Xi(1950), Xi(2250), Omega(2250) and Omega(2380) resonances, which have not been determined experimentally yet.
We present results for the strong widths corresponding to the $Σ_b\to Λ_b\, π$ and $Σ_b^{*}\to Λ_b\, π$ decays. We apply our model in Ref. Phys. Rev. D 72, 094022 (2005) where we previously studied the corresponding transitions in the charmed sector. Our non-relativistic constituent quark model uses wave functions that take advantage of the constraints imposed by heavy quark symmetry. Partial conservation of axial current hypothesis allows us to determine the strong vertices from an analysis of the axial current matrix elements.
We discuss the possibility and the description of bound states between $B$ and $\bar{B}^*$ mesons. We argue that the existence of such a bound state can be deduced from (i) the weakly bound X(3872) state, (ii) certain assumptions about the short range dynamics of the $D\bar{D}^*$ system and (iii) heavy quark symmetry. From these assumptions the binding energy of the possible $B\bar{B}^*$ bound states is determined, first in a theory containing only contact interactions which serves as a straightforward illustration of the method, and then the effects of including the one pion exchange potential are discussed. In this latter case three isoscalar states are predicted: a positive and negative C-parity $^3S_1-{}^3D_1$ state with a binding energy of $20\,{\rm MeV}$ and $6\,{\rm MeV}$ below threshold respectively, and a positive C-parity $^3P_0$ shallow state located almost at the $B\bar{B}^*$ threshold. However, large uncertainties are generated as a consequence of the $1/m_Q$ corrections from heavy quark symmetry. Finally, the newly discovered isovector $Z_b(10610)$ state can be easily accommodated within the present framework by a minor modification of the short range dynamics.
We investigate the $Λ(1520)$ photoproduction in the $\vecγ p \to K^+ Λ(1520)$ reaction within the effective Lagrangian method near threshold. In addition to the "background" contributions from the contact, $t-$channel $K$ exchange, and $s-$channel nucleon pole terms, which were already considered in previous works, the contribution from the nucleon resonance $N^*(2080)$ (spin-parity $J^P = 3/2^-$) is also considered. We show that the inclusion of the nucleon resonance $N^*(2080)$ leads to a fairly good description of the new LEPS differential cross-section data, and that these measurements can be used to determine some of the properties of this latter resonance.
We construct pi-pi amplitudes that fulfill exact elastic unitarity, account for one loop Chiral Perturbation Theory contributions and include all 1/Nc leading terms, with the only limitation of considering just the lowest-lying nonet of exchanged resonances. Within such scheme, the Nc dependence of sigma and rho masses and widths is discussed. Robust conclusions are drawn in the case of the rho resonance, confirming that it is a stable meson in the limit of a large number of QCD colors, Nc. Less definitive conclusions are reached in the scalar-isoscalar sector. With the present quality of data, we cannot firmly conclude whether or not the Nc=3 f0(600) resonance completely disappears at large Nc or it has a sub-dominant component in its structure, which would become dominant for a number of quark colors sufficiently large.
We present a model for weak CC induced nuclear reactions at energies of interest for current and future neutrino oscillation experiments. This model is a natural extension of the work of Refs.[1,2], where the QE contribution to the inclusive electron and neutrino scattering on nuclei was analyzed. The model is based on a systematic many body expansion of the gauge boson absorption modes that includes one, two and even three body mechanisms, as well as the excitation of Delta isobars. The whole scheme has no free parameters, besides those previously adjusted to the weak pion production off the nucleon cross sections in the deuteron, since all nuclear effects were set up in previous studies of photon, electron and pion interactions with nuclei. We have discussed at length the recent charged current quasi-elastic MiniBooNE cross section data, and showed that two nucleon knockout mechanisms are essential to describe these measurements.
In this paper we develop a formalism to evaluate wave functions in momentum and coordinate space for the resonant states dynamically generated in a unitary coupled channel approach. The on shell approach for the scattering matrix, commonly used, is also obtained in Quantum Mechanics with a separable potential, which allows one to write wave functions in a trivial way. We develop useful relationships among the couplings of the dynamically generated resonances to the different channels and the wave functions at the origin. The formalism provides an intuitive picture of the resonances in the coupled channel approach, as bound states of one bound channel, which decays into open ones. It also provides an insight and practical rules for evaluating couplings of the resonances to external sources and how to deal with final state interaction in production processes. As an application of the formalism we evaluate the wave functions of the two $Λ(1405)$ states in the $πΣ$, $\bar{K} N$ and other coupled channels.